1. Field of the Invention
The present invention relates to a method of manufacturing an optical fiber to be used in the field of optical communication systems, and particularly to a method of producing an optical fiber having no defect in the profile of its refraction index.
2. Description of the Prior Art
A known method of manufacturing optical fiber is often employed wherein the core and a cladding layer are first integrated into a single unit and a fiber is then obtained by heating and expanding this integrated unit. This method has the disadvantage that the boundary between the core and the cladding is not clearly defined, with a resulting loss in total reflection, and this method also has the disadvantage that defects in the refraction index profile result in the area near the center of the core.
Degradation of the total reflection efficiency directly affects the attenuation characteristic of the optical fiber, and particularly any defect in the refraction index at the area near the center of the core portion in the single mode fiber affects significantly the optical transmission.
Requirements which should generally be provided in an optical fiber of this kind are:
1. The optical transmissivity of the core glass which is used as the optical transmission line should be quite high.
2. In order to ensure highly efficient total reflection at the boundary of the core glass and the cladding glass the boundary must be clearly defined.
3. The refraction indices of the core and cladding glass should satisfy certain conditions: ##EQU1## where: n1: Refraction index of core glass
n2: Refraction index of clad glass PA0 a: Radius of core glass PA0 .gamma.: Optical wavelength
Generally, the following methods are well known for manufacturing optical fibers of the core glass and cladding glass combination type.
1. Double-crucible method:
The core glass material and cladding glass material are melted respectively in inner and outer crucibles forming a double-chambered crucible, and these materials are simultaneously extracted from a fine hole at the bottom of this double-crucible and formed into the fiber.
2. Rod-in tube method:
The core glass material is first formed into a rod and then inserted into a tube of the cladding glass material, and then they are both heated and fused in the form of an integrated solid rod. Thereafter, the rod is extended in the form of a wire, so that a fiber is obtained.
3. CVD Method (Chemical Vapor Deposition):
A starting material in gas form is thermally oxidized in a fused silica reaction tube which is heated and rotated by means of a lathe. The oxidized material is deposited on the inner surface of the fused silica tube. The deposit is then formed into the glass material. Thereby the thin film layers of the cladding glass and core glass can be formed repeatedly. Then, these layers are formed into a preform in the form of an integrated rod. This preform is expanded into the form of a wire by using a heat source to form the fiber.
However, in any of the above-mentioned known methods, the fiber is obtained by first forming the core and the cladding portion in an integrated body which is then heated and extended into the form of a wire. Therefore, such methods have the disadvantage that the boundary between the core and the cladding is not distinct so that the total reflection coefficient is degraded.
In addition, in the process (called the collapse process) using a CVD method where the deposited glass tube is collapsed into a solid rod, the glass tube is heated to a temperature as high as 2000.degree. C. Thereby, both SiO.sub.2 and the additive for controlling the refraction index (a dopant comprising GeO.sub.2, P.sub.2 O.sub.5 etc.) are vaporized. Moreover, this method also has another disadvantage, that the dopant (for example, GeO.sub.2) is more easily vaporized as compared with SiO.sub.2, so that the concentration of the additive (GeO.sub.2 in this case) for raising the refraction index is reduced at a very thin layer on the glass surface which becomes the core portion and results in a defect in the refraction index profile in the area near the center of the core portion, and simultaneously the distribution of the refraction index is deformed. Such condition is shown in FIG. 1 where the horizontal axis represents the radial direction in the fiber, with the core portion 1 and the clad portion 2. The vertical axis represents the refraction index. This figure graphically shows the refraction index and a defect A of the distribution of the refraction index.
Particularly, the core diameter of a single mode fiber is as small as several microns and therefore generation of any defects in the refraction index profile at an area near to the center of the core portion has a distinctively bad influence on the characteristic of optical transmission.
Therefore, currently desired is a method of manufacturing a fiber wherein the boundary of the core and clad portions is defined clearly, the total reflection coefficient is excellent and simultaneously no defect in the refraction index profile is generated in the area near the center of the core.